This invention relates to a clocked direct voltage converter with potential separation, in which a control circuit for controlling the transient times of a switching transistor is arranged on the primary side of the voltage converter, in which transistor a voltage proportional to an output quantity is chopped by means of a controllable switch. The chopped voltage is transmitted by a transformer to the primary side, is then rectified, filtered and then supplied to the control circuit.
Such a direct voltage converter is known from U.S. Pat. No. 4,495,554. In the known direct voltage converter, the controllable switch, by means of which the output voltage is chopped, is driven by the pulsatory voltage, which appears across the two connections of the secondary winding of the power transformer.
The control of the controllable switch--in U.S. Pat. No. 4,495,554 a bipolar transistor--by the unfiltered output voltage of the converter has the disadvantage that the closure and opening times of the controllable switch depend upon the instantaneous duty cycle--i.e. upon the opening and closure times of the switching transistor of the direct voltage converter. With a decreasing duty cycle, the control becomes increasingly more inaccurate. If the duty cycle is zero, as, for example, under no-load conditions at the output of the converter, in the known direct voltage converter information about the output voltage can no longer be transmitted to the primary side of the voltage converter. The control of the controllable switch by means of the unfiltered output voltage therefore limits the control range and the control accuracy of the known direct voltage converter.